Lava Tube Caves or Lava Tubes are
Primary cave
formed by large lava flows.
Molten rock (magma) that pours or oozes onto the Earth's surface
is then called lava.
The viscosity of the lava depends on its silica content.
Low-silica basalt lava has a low viscosity and can form fast-moving (15-50
km/h), narrow lava streams or spread out in broad sheets up to several miles
wide.

During long lasting eruptions, lava flows tend to become channeled into a
few main streams.
Overflow of lava from these streams solidifies quickly and supports the channel
walls, building natural levees or ramparts.
So actually the channel is created by the flowing lava itself, a self fupporting
process.

Lava streams that flow steadily in a confined channel for many hours to days may
develop a solid crust or roof and thus change gradually into streams within
lava tubes.
Because the walls and roofs of such tubes are good thermal insulators,
lava flowing through them can remain hot and flowing, much longer than surface
flows.
Lava can be transported for great distances from the eruption sites in the
tubes.

Lava produces big amounts of gas and steam, and while flowing through the tube
this produces a certain air pressure inside.
This keeps the tube open but also may press a hole into the covering crust,
which is still hot and rather soft.
Those venting holes are called hornitos (Spanish for little ovens) or
spatter cones.
The reason is simple, the gas pressure from below throws small pieces of lava
out of the venting holes, which form cones, up to 15m high, on top of the vent.
The lava in those cones is formed of layer above layer of spatters.

When the lava flow from the volcano ends, the lava will continue to move and
drain the tube downhill.
This leaves a partially empty tube which may be called lava tube cave.
Lava which is not able to flow out of the tube will remain in the form of
solidified pools, flows, or even waterfalls.

Lava tubes can be entered at the beginning and at the end of the tube.
Inbetween there are two kinds of openings or skylights, hornitos and roof
collapses.
Hornitos formed while the lava was still flowing, collapses are a result of
susequent erosion.
It is easy to differenciate between them, as hornitos do not have much collpsed
material at the ground, it was transported away by the flowinf lava stream.
Collapses are rather common, as the roof of lava tubes is rather thin, a result
of the way it formed.
Also the durability of this caves is rather low, they are generally gone in less
than 10,000 years.
Older tubes are extremely rare.

The most famous place for lava tubes on Earth is Hawai'i.
Because of the geography, the lava tubes on Big Island are the biggest in the
World.
Kazumura Cave
is 65,500m long [2010] and has a height difference of 1,102m [2010] from end to
end.
With this size it is the deepest cave of the USA and the 8th longest.
The second deepest cave of the USA is still a lava tube of Hawai'i, Huehue
Cave and many other caves in this list too.
Despite this superlatives, lava tubes are never more than a few meters below
surface!